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<h1 id="firstHeading" class="firstHeading">条件包含</h1>
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</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../preprocessor.html" title="c/preprocessor"> 预处理器</a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>#if</span><span>#ifdef</span><span>#ifndef</span><span>#else</span><span>#elif</span><span>#endif</span></span></strong></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="replace.html" title="c/preprocessor/replace"><span class="t-lines"><span>#define</span><span>#undef</span><span>#,## 运算符</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="error.html" title="c/preprocessor/error"><span class="t-lines"><span>#error</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="line.html" title="c/preprocessor/line"><span class="t-lines"><span>#line</span></span></a></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>预处理器支持源文件的各个部分的条件编译。此行为由 <code>#if</code> 、 <code>#else</code> 、 <code>#elif</code> 、 <code>#ifdef</code> 、 <code>#ifndef</code> 及 <code>#endif</code> 指令控制。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#if</b></code> <span class="t-spar">表达式</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#ifdef</b></code> <span class="t-spar">标识符</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#ifndef</b></code> <span class="t-spar">标识符</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#elif</b></code> <span class="t-spar">表达式</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#else</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#endif</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>条件预处理块以 <code>#if</code> 、 <code>#ifdef</code> 或 <code>#ifndef</code> 指令开始，然后可选地包含任意数量的 <code>#elif</code> 指令，然后可选地包含至多一个 <code>#else</code> 指令，并以 <code>#endif</code> 指令结束。任何内部条件预处理块都是分开处理的。
</p><p>每个 <code>#if</code> 、 <code>#elif</code> 、 <code>#else</code> 、 <code>#ifdef</code> 及 <code>#ifndef</code> 指令控制到首个不属于任何内部条件预处理块的 <code>#elif</code> 、 <code>#else</code> 、 <code>#endif</code> 指令为止的代码块。
</p><p><code>#if</code> 、 <code>#ifdef</code> 及 <code>#ifndef</code> 指令测试指定条件（见下文），如果指定的条件求值为 true ，则编译控制的代码块。这种情况下忽略后续的 <code>#else</code> 和 <code>#elif</code> 指令。反之，如果指定的条件求值为 false ，则跳过控制的代码块，并处理后续的 <code>#else</code> 或 <code>#elif</code> 指令（若存在）。在前一情况下，无条件编译 <code>#else</code> 指令控制的代码块。在后一情况下， <code>#elif</code> 指令的作用就像是一个 <code>#if</code> 指令：检查条件，基于结果编译或跳过控制的代码块，然后在后一情况下处理后续的 <code>#elif</code> 和 <code>#else</code> 指令。条件预处理块以 <code>#endif</code> 指令终止。
</p>
<h3><span class="mw-headline" id=".E6.9D.A1.E4.BB.B6.E6.B1.82.E5.80.BC">条件求值</span></h3>
<h4><span class="mw-headline" id=".23if.2C_.23elif"><code>#if, #elif</code></span></h4>
<p><span class="t-spar">表达式</span> 是常量表达式，仅使用<a href="../language/expressions.html#.E5.B8.B8.E9.87.8F.E5.8F.8A.E5.AD.97.E9.9D.A2.E9.87.8F" title="c/language/expressions">常量</a>和用 <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> 指令定义的标识符。任何非常量，未以 <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> 指令定义的标识符，求值为<i>0</i>。 
</p><p>表达式可以含有形式为 <code>defined</code> <span class="t-spar">标识符</span> 或 <code>defined (</code><span class="t-spar">标识符</span><code>)</code> 的一元运算符，若用 <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> 指令定义了该 <span class="t-spar">标识符</span> ，则返回 <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">1</span></span></span> ，否则返回 <i>0</i> 。若 <span class="t-spar">表达式</span> 求值为非零值，则包含该控制代码块并跳过其他。若所用的任何标识符不是常量，则用 <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> 替换它。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-c11"><td>
<p>注意： <code>#if cond1</code> ... <code>#elif cond2</code> 与 <code>#if cond1</code> ... <code>#else</code> 后随 <code>#if cond3</code> 有别，因为若 <code>cond1</code> 为真，则跳过第二个 <code>#if</code> 被跳过，且 <code>cond3</code> 不必为良式，而 <code>#elif</code> 的 <code>cond2</code> 必须是合法表达式。
</p>
</td>
<td><span class="t-mark-rev t-until-c11">(C11 前)</span></td></tr>
</table>
<h4><span class="mw-headline" id=".23ifdef.2C_.23ifndef"><code>#ifdef, #ifndef</code></span></h4>
<p>检查是否用 <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> 指令定义了标识符。
</p><p><code>#ifdef</code> <span class="t-spar">标识符</span> 本质上等价于 <code>#if defined</code> <span class="t-spar">标识符</span> 。
</p><p><code>#ifndef</code> <span class="t-spar">标识符</span> 本质上等价于 <code>#if !defined</code> <span class="t-spar">标识符</span> 。
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#define ABCD 2</span>
<span class="co2">#include &lt;stdio.h&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
 
<span class="co2">#ifdef ABCD</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#else</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1: no<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
 
<span class="co2">#ifndef ABCD</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: no1<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#elif ABCD == 2</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#else</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: no2<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
 
<span class="co2">#if !defined(DCBA) &amp;&amp; (ABCD &lt; 2*4-3)</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"3: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1: yes
2: yes
3: yes</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd><ul><li> 6.10.1 Conditional inclusion (p: 162-164)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999): 
</li></ul>
<dl><dd><ul><li> 6.10.1 Conditional inclusion (p: 147-149)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990): 
</li></ul>
<dl><dd><ul><li> 3.8.1 Conditional inclusion 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table></div>
</div>

<!-- 
NewPP limit report
Preprocessor visited node count: 1933/1000000
Preprocessor generated node count: 6226/1000000
Post‐expand include size: 38511/2097152 bytes
Template argument size: 6872/2097152 bytes
Highest expansion depth: 13/40
Expensive parser function count: 0/100
-->

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